This invention is related to isolation transformers that suppress high-frequency electromagnetic noise (hereafter called noise) transmitted through power transmission lines and/or signal transmission lines.
Micro-computers are being used in various fields such as information, communication, and other industries in addition to daily life etc. This is due to down-sizing, lower prices, and higher reliability of micro-computers driven by the continuous advancement of integrated circuits. However, integrated circuits are controlled by extremely small electric energy, and therefore, are subject to errors and damages caused by some noise transmitted from external sources. Such events will eventually lead to various inconveniences and/or accidents owing to malfunctioning or stopping of various equipment and devices that contain integrated circuits and of systems using them. Consequently, protection of electronic devices and equipment that contain densely packed and complicated circuits from noise-related troubles have become an urgent issue.
For suppression of noise-related troubles isolation transformers of electromagnetic-shield type have been used. The isolation transformers of electromagnetic-shield type have primary- and secondary coils isolated by approximately 20 μm-thick aluminum foils. The isolation transformers of electromagnetic-shield type have such attenuation characteristics of normal-mode noise as depicted in FIG. 11. Namely, in the frequency range of several hundred Hz to 1 MHz the attenuation increases generally mildly with the frequency to −50 dB. In the range from 1 MHz to 100 MHz it takes the form of an irregular saw-tooth wave, which is comprised by troughs and crests of various sizes between the maximum of −78 dB and the minimum of −24 dB.
The noise attenuation characteristic that occurs in the high-frequency region above a few MHz, and takes the form of an irregular saw-tooth wave, which is comprised by troughs and crests of various sizes is caused by stray capacitance appearing uniquely in each transformer; its origin can be traced to the multi-layer, multi-winding coils having numerous, irregular resonance circuits caused by complicated combinations of infinitesimally different inter-layer and inter-winding distributed capacitance as well as leakage inductances. Components with extremely complicated combinations such as multi-layer, multi-winding coils in transformers thus have random and complicated noise attenuation characteristics; as the crests have extremely low attenuation, this scheme cannot provide highly reliable isolation transformers. In order to improve the reliability of isolation transformers it is necessary to increase the attenuation in the high frequency region exceeding a few MHz and to fully decrease each amplitude of the saw-tooth wave comprised by troughs and crests of various sizes, suppressing the crests. As the irregular curves of the attenuation characteristic is unique to each transformer, the ideal suppression mechanism ought to commonly and effectively apply to any transformer. However, it has been impossible for the isolation transformers of electromagnetic-shield type to satisfy this condition.
Therefore, this inventor has developed two types of isolation transformers to resolve the aforementioned issue confronting the isolation transformers of electromagnetic-shield type: One of them was made public in the Japanese Patent No. 2,645,256, and, as shown in FIG. 10, it is an isolation transformer, which is characterized by shielding entirely both primary- and secondary coils by short-circuit rings 4 made of conducting thin films of thickness of 0.5˜100 μm.
Another is the type published in IEEE (U.S. Institute of Electrical and Electronics Engineers) Transactions on Electromagnetic Compatibility, Vol. 41, No. 3, August 1999. As shown in FIG. 9, this is an isolation transformer characterized by positioning in the vicinity of, or more specifically, in-between primary- and secondary coils a short-circuit ring 4 made of a conducting thin film, which has a thickness of approximately 7 μm or less (hereafter abbreviated as isolation transformer of short-circuit-ring type). As shown in FIG. 6 as an example, the core that forms a magnetic path between the primary coil 1 and the secondary coil 2, is manufactured by stacking together pieces of type E core and pieces of type I core of specified dimensions to specified thickness after stamping them out of an isotropic silicon steel sheet of thickness of 0.5 mm. Furthermore, the short-circuit ring 4 of conducting thin film is fabricated, as shown in FIG. 5 for an example, by cutting a ring, which is approximately as thick as the primary coil 1 and secondary coil 2, out of a rolled aluminum foil of thickness of 7 μm and by laminating it to a durable polyester film of thickness of 50 μm.
This short-circuit ring 4 made of metallic thin film with a large surface area functions as the tertiary coil connected to the primary- and secondary coils, respectively. Through this short-circuit ring 4 of conducting thin film flows the current induced by fundamental wave current flowing through the primary coil, its higher harmonics, and high-frequency noise from external sources. In this case, as the high-frequency components flow essentially at the surface of the conductor only owing to the skin effect, they flow around the short-circuit ring, even if the ring is thin. Therefore, being effectively attenuated by the resistivity of the short-circuit ring, it is hard for the high-frequency components to flow from the primary coil to the secondary coil. At the same time, since the resistivity of the short-circuit ring functions as a resistance commonly inserted to all of the resonance circuits, which are distributed in the form of a number of infinitesimally small irregularly present resonance circuits due to complex combinations of capacitance and leakage inductances irregularly distributed in the coils, the resistivity of the short-circuit ring has the effect of dramatically reducing the amplitudes of resonances. To sum up, in the isolation transformers of short-circuit ring, a short-circuit ring of conducting thin film or a short-circuit ring of conducting thin film laminated with a plastic film is used and the surface area of the aforementioned short-circuit rings is made approximately as large as that of the neighboring coil layers, and their thickness is made approximately identical to or less than the skin depth of the induced current generated by the skin effect in the high-frequency region, in which resonances should be suppressed.
Meanwhile, the currents induced by the fundamental wave, which are low-frequency components, are reduced in proportion to the cross sectional area of the conducting thin film of the short-circuit ring 4. However, as the short-circuit ring is made of a thin film of thickness of 7 μm, its cross section is small albeit its width. Therefore, the induced currents of fundamental-wave component that flows through the short-circuit ring is extremely small. As a consequence, by positioning this short-circuit ring 4 with a wide surface area in the vicinity of the primary- and secondary coils, respectively, an isolation transformer of short-circuit-ring type, which can eliminate or filter high-frequency noise, while keeping the loss of the fundamental-wave negligibly small, is provided.
Presented in FIG. 11 is an example of attenuation characteristics for the normal-mode noise of the isolation transformer of short-circuit ring type depicted in FIG. 9. Namely, in the range from a few hundred Hz to 1 MHz the attenuation increases generally mildly with the frequency to −60 dB, while in the range from 1 MHz to 100 MHz it has the form of an irregular saw-tooth wave, which is comprised by crests and troughs of various sizes between the maximum of −100 dB and the minimum of −53 dB. Furthermore, in the range from 100 MHz to 300 MHz it has the form of an irregular saw-tooth wave, which is comprised by crests and troughs of various sizes between the maximum of −72 dB and the minimum of −50 dB.
As is evident in FIG. 11, the attenuation characteristic curve of the isolation transformer of short-circuit ring type has a relatively flat portion with steep crests and troughs replaced with crests and troughs of smaller amplitudes. In comparison with isolation transformers of electromagnetic-shield type the isolation transformers of short-circuit-ring type show considerable improvements concerning the attenuation characteristic of normal-mode noise in the high-frequency region over 1 MHz. More specifically, as shown in FIG. 12, the isolation transformer of electromagnetic-shield type has the lowest attenuation of −24 dB, while as shown in FIG. 11 the isolation transformer of short-circuit-ring type has that of −53 dB, showing an outstanding improvement of 29 dB. The same trend exists for the highest points of attenuation, which is −78 dB for the isolation transformer of electromagnetic-shield type, while −100 dB for the isolation transformer of short-circuit-ring type, showing another outstanding improvement of 22 dB.
Furthermore, in the high-frequency range over 10 MHz considerable improvements are observed in the normal-noise characteristic. Namely, as evident in the region encompassed by thick dotted lines, the normal-mode attenuation characteristic in the range from 10 MHz to 100 MHz of the isolation transformer of the electromagnetic-shield type has the highest point of −78 dB and the lowest point of 40 dB, while that of the isolation transformer of the short-circuit-ring type has the highest point of −91 dB and the lowest point of 53 dB, showing considerable improvements of 13 dB at both the highest and lowest attenuation points.
Though not illustrated, the same trend exists for common-mode noise; isolation transformers of short-circuit-ring type show considerable improvements over isolation transformers of electromagnetic-shield type in the high-frequency region above a few MHz.
Though inside such components having extremely complicated combinations as multi-layer, multi-winding coils, there exist a number of resonance circuits caused by complicated combinations of infinitesimally different inter-layer and inter-winding distributed capacitance as well as leakage inductances, inside isolation transformers of short-circuit-ring type the effects of stray capacitance due to such resonance circuits are evidently reduced. Moreover, as in addition to increasing the attenuation rates in the high-frequency range above a few MHz the amplitudes of the irregular saw-tooth shaped waves with crests and troughs of various sizes were suppressed to the lowest possible level, the reliability of isolation transformers of the short-circuit-ring type has been considerably improved.
However, as evidently shown in FIG. 11, the suppression of amplitudes of the characteristic curve for the attenuation rate of normal-mode noise with irregular saw-tooth shaped waves with crests and troughs of various sizes in the high-frequency region above a few MHz is not sufficient yet. Therefore, there still remains a question with regard to the reliability of the conventional isolation transformers of short-circuit-ring type with a wide short-circuit ring of conducting thin film covering each surface of primary- and secondary coils and of the isolation transformers of short-circuit-ring type with a wide short-circuit ring of a conducting thin film positioned between and in the vicinity of primary- and secondary coils.